At present chemicals are added in fixed amounts every time a film is introduced into a photographic processing system. The amount is metered by a positive displacement pump, usually of the plastic bellows type, which has been adjusted and fixed by its manufacturers. This is wasteful as it makes no allowance for differences in overall exposure from film to film and more chemicals than necessary are often added. If relevant characteristics, such as the development rates and film density are monitored and used to calculate the correct amount of addition, not only may it be necessary to only replenish one chemical component of the process solution, but the amounts of any such addition may be very small. This raises a requirement for apparatus to dispense very small amounts of liquids or powders accurately controlled by weight, WO91/18326 and WO91/18327 disclose such apparatus.
U.S. Pat. No. 4,944,428 discloses apparatus for the automatic determination of a continuous bulk material throughput through a continuous balance.
U.S. Pat. No. 4,605,297 discloses apparatus for insuring application of a preselected amount of processing fluid to photographic film material in which a weight sensing device weighs a container of fluid and permits pumping of the fluid if there is an adequate amount of fluid contained in the container. A magnetic sensing arrangement also effects reversal of the pumping after a preselected length of material has had fluid applied to it.
U.S. Pat. No. 4,646,784 discloses apparatus for dispensing liquid chemicals such as photoresist from containers, in particular collapsible pouch-type containers, which containers have an inlet and a dispensing outlet, and a solenoid-controlled valve which is connected to the dispensing outlet by a plastic tube and which is opened to cause or allow discharge of fluid from the container through the dispensing outlet. The container is suspended from an electric weight detector which is arranged to switch off the solenoid valve automatically when it detects the container is empty.
The apparatus disclosed by each of the prior art references discussed above has a flexible pipe connected to the dispensing outlet of the container and is thereby vulnerable to inaccuracies due to stiffness, varying weight effects and hysteresis.